Field of the Invention
The present invention relates in general to a cold formable spring steel wire excellent in cold cutting capability and fatigue properties and a manufacturing process thereof, more specifically, to a spring steel wire having a superior cold cutting capability required for the manufacture of springs and good fatigue strength (endurance in air) as a significant spring property, and a manufacturing process of the spring steel wire. Although a spring steel of the present invention is useful in manufacture of springs for use in diverse fields inclusive of the transportation field such as automobiles, ships and the like, and the industrial machinery, it is assumed that the present invention steel is used as a material of parts in an automobile as a typical example.
Description of the Related Art
The chemical compositions of spring steels are specified in JIS G 3565 to 3567, JIS G 4801 and the like. By use of these spring steels, various cold formable springs can be manufactured by the steps of, that is, after hot rolling a steel material satisfying the above-described chemical composition: (A) drawing the rolled material to a specified diameter without performing an annealing (softening) process; (B) drawing the material after annealing (softening); and (C) cutting the surface after annealing (softening), and heating and drawing. When the drawing process is completed as above, a spring steel wire is quenched and tempered, being formed into a spring with a predetermined tensile strength. The spring steel wire is then wound by a cold forming coiling machine and each piece is generally cold cut by a shear. Furthermore, in order to remove distortions in the wound springs, annealing is performed at a low temperature, and the surface of the wires is hardened through shot peening and/or nitriding.
There have been strong demands toward the enhancement of the stress of a spring as a part of measures of achieving small and light springs in order to reduce exhaust gas or fuel consumption. For example, there is required of a high strength spring steel wire of which tensile strength after quenching and tempering is 2000 MPa or greater.
As a technology related to the cold formable steel wire, Japanese Patent Gazette No. 3453501 suggests that balance of the composition should be controlled in order to obtain steels for cold winding with small residual stress generated during a bending process.
Meanwhile, as the strength of a spring increases, the sensitivity against defects is generally increased. Also, even insignificant defects that do not necessarily affect fatigue properties, such as, minor defects caused during transportation or wire drawing defects present on the surface of the spring steel wire, accelerate propagation of fatigue cracks around the location during a cold shear cutting process. However, controlling the balance of the composition as disclosed in the above-described technique is not sufficient to suppress cracks at the time of cold shear cutting.
Many technologies for suppressing cracks during cold shear cutting have been suggested. One of them is Japanese Patent Gazette No. 3627393, which indicates that the cause of cold shear cracks is high notching susceptibility. Thus, to decrease the notching susceptibility, it teaches to control average particle size of carbides or volume ratio of carbides in the steel. However, according to this technology, Si content is limited to below 1.5 mass % because Si is known to deteriorate workability. Ironically though, it is very difficult to achieve a tensile strength higher than 2000 MPa with that amount of Si. Therefore, the above-described technology cannot increase the cold cutting capability in a high strength region nor enhance the fatigue property that is a significant feature for a spring.